The present invention relates to an outboard motor particularly provided with an improved exhaust structure.
Engines of outboard motors include an engine, generally of water-cooling type, having a cylinder block in which a plurality of cylinders are arranged in a vertical direction. In a cylinder head connected to the cylinder block, intake ports and exhaust ports are formed for the respective cylinders. Exhaust gas discharged from the exhaust ports is collected into a common exhaust passage and then discharged from the outboard motor.
Since the exhaust passage is heated to a very high temperature by the exhaust gas, a cooling water passage is formed around the exhaust passage to cool the exhaust passage by means of cooling water. Such a cooling water passage is provided for members of the engine, such as cylinder block, an engine holder and an oil pan.
However, in a conventional structure of the outboard motor, in some cases, the exhaust gas is not sufficiently cooled even by the location of a cooling water passage to a member (or members) such as cylinder block, engine holder and oil pan through which an exhaust passage passes. When the exhaust gas having a high temperature from the engine is discharged from the discharge passage into the drive shaft housing, an outer wall of the drive shaft housing is heated by the exhaust gas and a temperature thereof is increased and, hence, calcium contained in a sea water, for example, pumped up as cooling water may adhere to the outer wall of the drive shaft housing, thus being inconvenient.